1. Field of the Invention
The invention relates generally to the field of imaging subsurface formations while drilling wellbores therethrough. More specifically, the invention relates to instrument structures and signal processing techniques for such imaging that can provide enhanced resolution and formation identification.
2. Background Art
Instruments are known in the art for creating a representation of a visual image of subsurface formations while a wellbore is being drilled through such formations. Such instruments include devices that measure formation resistivity, acoustic wave properties, formation density, neutron porosity, neutron capture cross section and nuclear magnetic resonance properties, among others. Typically one or more of such sensors is mounted in one or more “drill collars” (a drill collar being a thick-walled segment of drill pipe) coupled within a drill string. The drill string is a long pipe extending from the surface to the bottom of the well and is used to suspend and rotate a drill bit to lengthen the wellbore by drilling the subsurface formations. As the drill string moves along the wellbore, whether during drilling or during pipe movements subsequent to drilling (e.g., reaming, washing, tripping) measurements such as the foregoing may be made at various rotary orientations of the drill string. The measurement value and the rotary orientation may be recorded in suitable storage devices in the instrument and/or may be transmitted to the surface using various forms of drill string telemetry.
A limitation to the imaging techniques known in the art is that they generally are limited as to the distance in the formation that can be examined or imaged. There exists a need for formation imaging devices that can determine the equivalent of visual properties of formations at substantially greater distances from the wellbore than the capabilities of instruments known in the art.